1. Field of the Invention
This invention relates to an ink jet printer head of the type having ink reservoirs wherein at least one of the walls of each reservoir is made of a piezoelectric ceramic material and is activated by an electrode.
2. Description of the Related Art
Ink jet devices making use of piezoelectric ceramic elements are known and have been hitherto proposed including, for example, drop-on-demand type ink jet devices. The device is arranged so that a piezoelectric ceramic element has a number of grooves, each individual grooves having the capacity to deform due to the piezoelectric ceramic material. When the capacity or volume of a groove is reduced, ink in the groove is jetted from a corresponding nozzle in the form of droplets. When the capacity is increased, the ink is introduced from an ink introducing pipe into the groove. A multitude of nozzles are provided adjacent to one another, so that when ink droplets are jetted from given nozzles according to given printing information, a desired letter or image is formed on a paper sheet provided in face-to-face relation with the nozzles.
Referring to FIG. 1, a known ink jet device is shown. The device includes a piezoelectric ceramic element 1 having a plurality of grooves 12 wherein the element 1 is polarized in the direction of arrow 4. The device also includes a cover plate 2 made of a ceramic or resin material bonded with the element 1 through a bonding layer 3 such as an epoxy adhesive, thus defining the plurality of grooves 12 as ink passages. Individual ink passages have an elongated shape with a rectangular section and each includes side walls 11 extending over the entire length of the ink passage. The side walls 11 are formed with a metal electrode 13, to which a drive electric field is applied, on opposite surfaces thereof extending from the top of the side wall in the vicinity of the adhesive layer 3 at the apex of the side wall 11 toward the central portion of the side wall 11. Each electrode 13 is covered with a protective film 20 as shown. Ink is filled in all of the ink passages during operation.
The operation of the device is illustrated with reference to FIG. 2, which is a sectional view of an ink jet device. In the ink jet device, if a groove 12 is, for example, selected according to given printing information, a positive drive potential is quickly applied between the metal electrodes 13e and 13f, and metal electrodes 13d and 13g are connected to ground. By this arrangement, a drive electric field acts on the side wall 11b along the direction of arrow 14b and on the side wall 11c along the direction of arrow 14c. Since the drive electric fields 14b and 14c are crossed at right angles with respect to the direction 4 of polarization, the side walls 11b, 11c are rapidly deformed in the direction of inside of the groove 12b owing to the piezoelectric perpendicular slide effect. The deformation contributes to the reduction in capacity of the groove 12b, leading to the quick increase of pressure exerted on an ink. This eventually generates a pressure wave in the groove 12b, so that ink droplets are jetted from a nozzle 32 of FIG. 3 in communication with the groove 12b. If the application of the drive potential is gradually stopped, the side walls 11b and 11c are returned to the respective positions prior to the deformation, and, thus, the ink pressure within the groove 12b is lowered. Accordingly, fresh ink is supplied from an ink inlet port 21 of FIG. 3 through a manifold 22 into the groove 12b.
In conventional ink jet devices, a drive potential may be applied, prior to the jetting operation, in a reverse direction as described above to initially supply the ink. Subsequently, the drive potential is abruptly stopped, by which the side walls 11b, 11c are, respectively, returned to the original positions thereof, thereby causing the ink to be jetted.
Next, reference is made to FIG. 3 showing a perspective view of an ink jet device to illustrate arrangement and fabrication of the known device. The piezoelectric ceramic element 1 is formed with grooves 12 according to cutting by a thin disk-shaped diamond blade or the like. The grooves 12 are arranged parallel to one another and have substantially the same depth throughout the piezoelectric ceramic element 1 but are gradually smaller in depth as they approach opposite end faces 15. In the vicinity of the end faces 15, a shallow, parallel groove portion 16 is provided. The metal electrodes 13 are formed on the inner side walls of each groove 12 according to known techniques such as sputtering. The protective layer 20 is formed on the inner surfaces of the grooves 12 by a dry or wet method so as to cover the electrodes 13 therewith.
A cover plate 2 made of a ceramic or resin material is subjected to grinding or cutting to make an ink introducing port 21 and a manifold 22. The piezoelectric ceramic element 1 and the cover plate 2 are bonded by an epoxy adhesive or the like such that the side of the element 1 having the grooves 12 and the side of the plate 2 having the manifold are facing each other. A nozzle plate 31 having nozzles 32 provided in correspondence with the respective grooves 12 is bonded at one end face of the piezoelectric ceramic element 1 and the cover plate 2. A substrate 41 having a pattern 42 of conductive layers positioned to correspond to the respective grooves 12 is bonded, preferably by an epoxy adhesive, to a side opposite to the groove 12, or the bearing side of the element 1. Metal electrodes 13 formed at the bottom of each shallow groove portion 16 of the grooves 12 are connected to the pattern 42 of conductive layers through conductive wires 43 through wiring bonding.
Referring to FIG. 4, a block diagram of a known control unit is shown to illustrate an arrangement of the control unit. The conductive layers of the pattern 42 on the substrate 41 are individually connected to an LSI chip 51, and a clock line 52, a data line 53, a voltage line 54 and a ground line 55 are, respectively, connected to the LSI chip 51. The LSI chip 51 determines which nozzles are used to jet ink droplets based on data appearing on the data line 53 on the basis of on a continuous clock pulse passed from the clock line 52. Then, a voltage V of the potential line 54 is applied to selected conductive layers of the pattern 42 connected to the corresponding metal electrodes 13 of the grooves 12 to be driven. At the same time, conductive layers of the pattern 42 connected to the metal electrodes 13 other than the applied electrodes are applied with a voltage of 0 V from the ground line 55.
In the ink jet printer head having such an arrangement or mechanism as set forth hereinabove, a protective film 20 is provided to ensure insulation protection of individual electrodes 13 and to prevent the electrodes from being corroded. The protective film 20 is preferably made of an inert inorganic passive state film having an alternately built-up structure of silicon nitride (SiN.sub.x) and silicon oxynitride (SiON).
However, the protective film for insulation protection of the electrodes of the ink jet head influences performance of the electrode due to its thickness. The film affects characteristics such as insulation breakdown characteristics, adhesion, stability and the like, and deformation characteristics of jetting ink. If the film thickness is too small, the insulating properties are poor. If the thickness is too large, deformation characteristics are worsened, with attendant drawbacks such as cracks and film separation. The failures of the protective film relate to the stability in quality of the printhead. Since no limitation is placed on the thickness of the protective film in the prior art, the characteristics of the protective film are not uniform. Thus, problems occur in the quality of the printhead causing poor performance with a lowering of yield.
Moreover, in the prior art, no limitation is placed on how to form the protective layer for the coverage. This also leads to failures in head-to-head uniformity of protective film characteristics, quality and stability, resulting in a lowering of yield.
Likewise, no limitation is placed on the type of protective layer in the prior art. This leads to failures in head-to-head uniformity of protective film characteristics, quality and stability, resulting in a lowering of yield.